That depends on the circuit.
What is the wattage of your microwave?
Is this a 15 or 20 amp breaker?
Is the guage of wire correct for the breaker on this circuit?
A 15amp breaekr will have 14AWG wire a 20amp breaker will have 12AWG wire.
You will not want to put a 15 amp GFI on a 20 amp circuit

You are correct about Canada. The receptacle has to match the circuit. A 20 amp circuit has to have 20 amp receptacles. They can be the T slot version that accepts 15 amp plugs or they can be the dedicated 20 amp version with the horizontal only slot.

Hi - The GFCI outlet in the bathroom died - it is 3-prong w/red & black buttons in center - have no idea if it is a 15 or 20amp. It is the only outlet in the bathroom. Other switches are for the lights and fan. Do I use a 15 or 20amp and what is the difference??? Thanks...

The red/black buttons in the middle are the 'test'/'reset' buttons.
If by 'died' you mean tripped, press the 'reset' button to restore power to the outlet.
If the 'reset' button won't hold down then turn off the circuit breaker to the outlet (and for this will have to verify no power with a voltage meter or sensor), remove the outlet, verify no voltage, what amp it is and replace it with same amperage.

Use a 15 amp GFCI unless you use a lot of power out of this outlet. If you figure you use more than 12 amps then consider a 20 amp GFCI. But, you will need to verify #12 or larger wire before you do. Most GFCI receptacles are fine to be 15 amp rated and there is nothing wrong with using them on 15 or 20 amp circuits unless they are going to be submitted to very high load.

Use a 15 amp GFCI unless you use a lot of power out of this outlet. If you figure you use more than 12 amps then consider a 20 amp GFCI. But, you will need to verify #12 or larger wire before you do. Most GFCI receptacles are fine to be 15 amp rated and there is nothing wrong with using them on 15 or 20 amp circuits unless they are going to be submitted to very high load.

If the plug is rated for 15 amps and you plug in two items that add up to more than 15 amps but less than 20, you have a problem. Far safer to use the 20 amp plug and there might be rules on this .